Storage clusters and other storage systems store user data in various types of storage memory, such as hard drives, tapes, and solid-state storage. Some storage systems use a pipelined architecture, in which data is written to volatile memory and later transferred from volatile memory to nonvolatile memory, or from non-persistent memory to persistent memory. This may allow additional processing, or take advantage of differences in write speeds or write latency of various types of memory. Defining data coherency mechanisms in storage systems, so as to allow for data recovery in case of power failure, operating system or application crashes, poses challenges, some of which are general and some of which are unique to system architecture.
It is within this context in which various embodiments arise.